The present invention relates to a readout chip for single photon counting.
X-ray diffraction patterns are useful in the analysis of both organic and inorganic crystallographic structures. Especially organic materials, such as protein crystals, impose stringent requirements on x-ray detectors, particularly where the x-ray source is high flux synchrotron radiation that enables and requires, due to the radiation damage, an experiment to be done rapidly. Furthermore, an important and developing field is time-resolved diffraction experiments using synchrotron radiation, such as crystallography and/or powder diffraction analysis. Monitoring a time-dependent reaction in a sample, i.e. a crystal or a powder, can elucidate the time-dependent crystal/molecular changes that occur in a chemical reaction. High time resolution is often critical in such monitoring.
A fast single photon-counting imaging device for the above-mentioned purposes has been disclosed in the international patent application WO 2004/064168. This reference teaches in detail a photon-counting imaging device for single x-ray counting which comprises:                a) a layer of photosensitive material;        b) a source of bias potential;        c) a source of threshold voltage supply;        d) an N×M array of photodetector diodes arranged in said layer of photosensitive material; each of said photodetector diodes having a bias potential interface and a diode output interface, said bias potential interface of each photodetector diode being connected to said bias potential;        e) an N×M array of high gain, low noise readout unit cells, one readout unit cell for each photodetector diode;        f) each readout unit cell comprising an input interface connected to said diode output interface, a high-gain voltage amplifying means comprising a comparator unit, a digital counter unit, comprising a digital counter, and a digital counter output interface connected in series, each digital counter unit counting an output signal of the comparator unit; said digital counter output signal is proportional to the number of absorped incident photons in the respective photodetector diode,        g) a multiplexing means comprising a row select and a column select allowing to access each of the readout cell units, i.e. to read out the digital data as actually stored in the digital counter to the digital counter output interface;        h) each digital counter output interface connected to an output bus;        i) said output bus being connected to a data processing means controlling the multiplexing means.        
According to these measures, a photon counting imaging device is created having an architecture of the readout circuitry that allows to be tolerant with respect to a local defect of a detector diode and/or readout unit cell and that allows to control and redesign the program and/or the status of each detector diode and/or readout unit cell in order to achieve a fast photon counting imaging device.
However, even for this fast photon counting imaging device it is desirable to increase the speed of the data readout. So far, the counter in the reference cited above has been designed as a pseudo-random counter, such as an 18 bit shift register with an XOR feedback. The data from this counter is serially transmitted on one digital output pin during readout. For an 18 bit shift register a readout time of at least 1+18+1 cycles is required.